Sorting of axons according to their target destination is essential for axonal guidance in the developing nervous system. Intercellular interactions have been shown in virtually all experimental models to be critical in axon guidance, but a full understanding requires knowing the cellular and molecular underpinnings not only of neuron-neuron interactions but also of glia-neuron interactions. This proposal focuseson reciprocal glia-neuron interactions underlying the sorting of olfactory axons during development of the olfactory pathway, especially on the role of glial cells in regulating specific signaling pathways. During development, the axons of olfactory receptor neurons (ORNs) must shed the axonal associations they established in the periphery and form new odor response-specific relationships. In the moth Manduca sexta, ORN axons sort in a discrete, highly accessible, glia-rich region of the nerve that extends from the sensory epithelium to the brain. Development is initiated by ORN axons;later arriving axons abruptly change their growth pattern and their associations as soon as they enter this "sorting zone". Importantly, the axons must interact with the glial cells to sort properly into fascicles destined for glomeruli in the olfactory lobe. Thus the sorting zone of Manduca is an especially advantageous preparation to ask questions about glia-axon signaling mechanisms underlying axon sorting and fasciculation. The studies use parallel in vivo and in vitro approaches to test the hypothesis that glial cells regulate the balance of specific positive (e.g., growth-promoting or adhesive) and negative (e.g., growth-inhibiting or repulsive) cues that guides sorting of ORN axons. EM, confocal and live- cell microscopy will be used to assay the contributions of the cell adhesion molecules Fasciclin II and Neuroglian, the receptor tyrosine kinases EGFR and FGFR, and the EphR/ephrin signaling systemto axonal sorting. Manduca's olfactory system has sufficient complexity and similarity to the mammalian systemto be a useful model in which to study sorting. Relevance : The long term goal is to elucidate the mechanisms of glia-neuron interactions that are likely to be common to olfactory development and development of many other CNS pathways. Understanding these reciprocal neuron-glia interactions will contribute to the foundation needed to conquer devastating developmental abnormalities of the nervous system and inadequate regenerative attempts following injury.